Armature reaction is a basic problem both for dynamos using commutators (that is D.C. or single phase A.C. machines) and in synchronous motors and generators. The flux resulting from the armature electromotive force causes a distortion and, because of saturation effects, a weakening of the exciter field. The latter in turn causes a loss of voltage in the armature circuit, that is a decrease in the output of the machine. It is, therefore, highly desirable to keep the armature reaction within reasonable bounds. For this reason, many conventional machines are built with an air gap between the main pole and the armature which is substantially larger than would be required from the purely mechanical point of view. However, under these conditions, a greater exciting current is required, which in turn causes an increase in the size of the machine. This of course also increases its cost.